In recent years it has been increasingly important to be able to store and manipulate large amounts of data. Many attempts have been made to develop materials and techniques which would provide means of storing increasing amounts of data with the use of minimal amounts of material. One of the most promising areas in this regard has been digital optical data storage.
Digital optical data storage allows for the recording and processing of information via light. An example of such a data storage device is the compact disc. Thus, developments have provided us with the ability to store information on optical discs. However, this device merely provides the capability of imprinting information or storing information once on the material, which can then be read almost indefinitely.
Although these "write-once" and "read-only" optical discs are available, there remains a need for a truly reversible optical recording media which provides the ability to read, write, erase and then re-record information on the medium. Materials investigated for these purposes have included alloys of rare earth elements and transition metals which allow for combining magnetic and optical recording techniques. Falco, C. M. et al., Mater. Res. Soc. Symp. Proc., Vol. 151; Greidanus, F.J.A.M. et al., Adv. Mater, 1:235 (1989). However, organic materials for these purposes have been the increasing focus of research.
Photoreversible compounds, where the reversible switching process is based on photochemically-induced interconversions, play an important role in the search for the ideal optical switch for optical data storage material. Photochromism, which is defined as the reversible change induced by light irradiation between two states of a molecule having different absorption spectra, can be the basis for such a switch. Although the photochromism of a given material or compound may be bistable, such a compound material must also satisfy other requirements to be commercially or practically suitable for the optical storage of data. Thus, it becomes necessary to also find a molecule which will not interconvert between its two states within a large temperature range, so as to allow for stable storage of information. Along these lines, such a material must also be capable of withstanding thermal and photochemical degradation to side products and be resistant to change should it become necessary to use such a material to perform several cycles of writing and erasing.
The two forms of the compound or material should also be easily interconvertible, avoiding the necessity of long irradiation times. Furthermore, each state must be readily detectable.
Attempts to find an organic material suitable for these purposes have thus far fallen short of finding a practical solution. As stated above, many attempts to procure the ideal optical data storage material have been made. These include the use of cis-trans isomerizations, photocyclizations, photochromic keto-enol tautomerism, as well as chirochromic switches. Feringa, B. L. et al. Tetrahedron, 49(37):8267-8310 (1993); Zhang, M. et al., J. Am. Chem. Soc., 116:4852-4857 (1994). Again, each of these attempts to solve the problem of truly reversible optical data storage has been met with their own set of problems. For example, photocyclizations have been found to result in undesirable side reactions which interfere with the material's use as an optical data storage material. Furthermore, several of these compounds exhibit low thermal stability. Feringa, B. L. et al., Tetrahedron, 49(37):8267-8310 (1993).
One approach to attaining an optical switch has involved the use of photoresolution in which an organic molecule which is capable of existing in two mirror image forms can be switched between these forms by the use of circularly polarized light. In this situation the switch position can be read using polarized light with no change in the readout wavelength since both mirror image forms, i.e. enantiomers, have identical spectra. This approach has been the subject of considerable research effort, combined with efforts to utilize this approach in a liquid crystal. However, the result of this effort has not borne fruit as photoresolveable organic compounds used thus far, have yielded only very small enantiomeric excesses when exposed to circularly polarized light. Even the amplification attained by using these photoresolved compounds as dopants in nematic liquid crystals, causing conversion to cholesteric states, was not adequate to attain an optical switch. This problem has been discussed in Zhang, M. et al., J. Am. Chem. Soc., 116:4852-4857 (1994). As a result efforts have been modified in the direction of searching for light altering changes to chiral diastereomers instead of enantiomers. Even here though the potential for development of an optical switch is limited by the excess of one diastereomer over the other caused by the irradiation.
Accordingly this invention provides a solution to the general problem of the excess of one bistable photoproduced state over the other by attaching the photoswitchable group to a stiff helical polymer which has the inherent characteristic of chiral amplification within its structure with an additional chiral amplification added thereto when the polymer forms or is part of a liquid crystal. Stiff polymers form liquid crystals and are compatible with the director fields of liquid crystals formed from other substances.